A liquid crystal display device utilizing a transverse electric field system which applies an electric field to a liquid crystal in a parallel direction with a substrate has a wide viewing angle and is the standard for a large screen liquid crystal display. Such technologies have been proposed, for example, by Japanese patent laid-open publication Nos. 10-55000, 10-325961, 11-24104, 10-55001, 10-170939, and 11-52420, and have been improved to solve problems such as vertical crosstalk.
Numerous technologies have been proposed by liquid crystal makers including a technology in which a photolithography spacer is utilized to improve the contrast of a transverse electric field type liquid crystal display system. For example, such a technology has been proposed by Japanese patent laid-open publication Nos. 2000-199904 and 2000-19527. The majority of them utilize a photolithography technology to establish a spacer on a color filter substrate. The Japanese patent laid-open publication Nos. 2000-19527 and 2000-199904 show an idea of producing an electric field of a video signal line in a vertical direction of a substrate rather than a horizontal direction in order to suppress the vertical crosstalk. This requires the dielectric constant of the photolithography spacer to be larger than the dielectric constant of the liquid crystal. The Japanese patent laid-open publication No. 2000-19526 also proposed a photolithography spacer in which the dielectric constant is larger than that of the liquid crystal.
The Japanese patent laid-open publication No. 2001-209053 proposes a photolithography spacer in a vertical electric field system utilizing dielectric material with a smaller dielectric constant than that of the liquid crystal along a video signal line to cover the video signal line in order to lower the waveform distortion. According to this patent publication, the liquid crystal cell is constructed by creating a vacuum space inside the liquid crystal cell, then injecting the liquid crystal in the space through an injection opening using the atmospheric pressure. In this liquid crystal injection method, a batch process is used in which several hundred cells are processed at the same time to produce a large liquid crystal panel.
Japanese patent laid-open publication Nos. 2002-258321 and 2002-323706 teach a structure using a dielectric material of a smaller dielectric constant than that of the liquid crystal along the video signal line to cover the video signal line and placing a transparent conductive material along the video signal line to improve the pixel aperture ratio as well as to prevent signal delay.
FIG. 3 is a flow chart showing a typical production process in the conventional technology for producing a TFT (thin film transistor) array substrate (active matrix substrate) of the transverse electric field type liquid crystal panel. This production process includes four-step photomasking processes using the conventional halftone exposure technology. FIGS. 36A-36F are cross sectional views showing the structural developments in accordance with the production flow of FIG. 3 using the four-step photomasking technology.
In the conventional production process of FIG. 3, gate electrodes of thin film transistors and common electrodes are formed at the same time in step S11. Then, at step S12, thin film transistors are separated from a semiconductor layer and source electrodes and drain electrodes of the thin film transistors are formed using the halftone photomask exposure. In step S13, contact holes for gate terminals, data terminals, pixel drain portions, and transistor circuits for electrostatic protection are created. Then, at step S14, gate terminals, data terminals, transparent conductive pixel electrodes are formed.
In the cross sectional views of FIGS. 36A-36F, a numeral 6 denotes an area on a positive photoresist layer after development where UV exposure is blocked, a numeral 7 denotes an area on the positive photoresist layer after development where the UV exposure is made through the halftone (translucent) photomask, a numeral 9 denotes a gate insulation film, a numeral 10 denotes a thin film semiconductor layer (non-doped layer), a numeral 11 denotes a thin film semiconductor layer (doped layer, i.e., ohmic contact layer), a numeral 15 denotes a scanning line, a numeral 50 denotes a scanning line terminal, a numeral 51 denotes a video signal line, a numeral 54 denotes a scanning line drive circuit contact electrode, a numeral 64 denotes a drain electrode of the thin film transistor, and a numeral 65 denotes a transparent pixel electrode.
Prior to the start of the processes of FIGS. 36A-36F, the scanning lines 15 and the scanning terminals 50 are formed on a glass substrate (not shown). In FIG. 36A, the gate insulation film 9, the thin film semiconductor layer (non-doped layer) 10 and the thin film transistor ohmic contact layer 11 are respectively deposited by, for example, a CVD plasma device. The positive photoresist 6 is coated and the halftone exposure is conducted so that the thicker positive photoresist 6 and the thinner positive photoresist 7 are created. In FIGS. 36B and 36C, through a dry etching process, the thin film transistors are separated from the semiconductor layer. In FIG. 36D, the drain electrode 64 of the thin film transistor and the video signal line 51 are formed by further conducting the etching process. In FIG. 36E, through the dry etching, contact holes are created over the scanning line terminals 50. In FIG. 36F, the scanning line drive circuit electrodes 54 and the transparent pixel electrodes 65 are formed.
The conventional transverse electric field type liquid crystal panel utilizes common electrodes placed at both sides of the video signal line to shield the electric field caused by the signal video line. In order for this construction to completely solve the problem involved with the vertical crosstalk, it is necessary to design the width of the common electrodes to be at least 1.5 times larger than that of the video signal line, hence resulted in a reduction of the pixel aperture ratio.
It is possible to reduce the vertical crosstalk by collecting the electric force lines of the electric field produced by the video signal line to the photolithography spacer. This can be done by placing a black mask made of thin film conductive material (chromium oxide layer and chromium metal thin film layer) at the side of color filter and setting the electric potential of the black mask to that of the common electrodes, and creating a photolithography spacer that is placed in an elongated fashion at the same direction as the video signal line by an insulation material that has a dielectric constant larger than that of the liquid crystal. However, in this method, because the material of large dielectric constant is used, the capacitance between the black mask and the video signal line is increased, hence the video signal waveform is delayed and distorted, which is not appropriate for a large screen liquid crystal panel.
As disclosed in Japanese patent laid-open publication No. 11-24104, it is possible to almost completely shield the video signal line by constructing a passivation layer on the video signal line and placing a shielding electrode thereon along the video signal line. However, because this construction utilizes a very thin passivation layer with a thickness in the range between 0.3 micrometer and 1 micrometer, and the passivation layer made of silicon oxide or silicon nitride has a relatively large dielectric constant of 4-6, the capacitance between the shielding electrode and the video signal line increases. This causes the signal waveform to be delayed and distorted and is not appropriate for a large screen liquid crystal display panel.
Japanese patent laid-open publication No. 2001-209053 discloses a photolithography spacer constructed in a very thin manner that surround the video signal line using a dielectric material of a small dielectric constant so that the capacitance between the common electrodes on the side of the color filter and the video signal line can be decreased. This technology, however, utilizes a conventional method of injecting the liquid crystal through an injection opening. Thus, the thin and long photolithography spacers cause the liquid crystal to be injected at a very slow speed, which severely decreases the production efficiency.
Japanese patent laid-open publication Nos. 2002-258321 and 2002-323706 disclose a structure which utilizes a dielectric material that has a smaller dielectric constant than that of the liquid crystal to cover the video signal line and places a transparent conductive material along the video signal line so that the pixel aperture ratio can be improved and the video signal line delay can be prevented. However, with this construction, it is not possible to produce the liquid crystal cell and the spacer at the same time. Therefore, an additional photolithography process has to be performed to produce the photolithography spacers. This causes the production processes to be more complicated and costly.
The implementation of the technology disclosed in Japanese patent laid-open publication No. 2002-258321 is not enough to produce a transverse electric field type liquid crystal panel with high contrast and low light leakage. When an angle of the bumps of the dielectric material with a small dielectric constant that cover the video signal lines along the video signal lines is larger than 40 degrees, the conventional technology of rubbing treatment using rubbing cloth causes areas with alignment defects due to the sideways slip caused at the tapered portions of the bumps in the direction of the movement at the tips of hairs of the rubbing cloth or areas on the inclined surfaces of the bumps where the hair tips of the rubbing cloth cannot reach.